To gain insight into the roles of HCF-1 in cell cycle control, biochemical and genetic approaches will be combined to study the interaction between HCF-1 and the Myc/Max/Mad transcription factor network that plays a central role in controlling cellular proliferation, differentiation, and apoptosis. The specific aims are: (1) To probe HCF-1-Mad interactions; (2) To determine whether HCF-1-Mad interactions affect the transcription properties of Mad; and (3) To study the influence of HCF-1-Mad interaction on cell proliferation and differentiation. Specifically, immunoprecipitation will be used to analyze the association of HCF-1 with Mad, and whether this association is affected by other proteins, such as Max, Myc, and Sin3. Electrophoretic mobility shift assays will be used to examine the DNA-binding activity of the HCF-1-Mad complex, and in vivo transcription activity assays will provide information about roles of HCF-1 in modulating the activity of Mad. Small interfering RNAs will be used to deplete endogenous Mad and study the activities of mutant forms of Mad that are defective in HCF-1 association. These studies will reveal the biological roles of HCF-1 and Myc-Max-Mad network in cell proliferation, differentiation, and cancer progression.